It is well known to electrochemically etch the surface of an aluminum foil used as an electrode of an electrolytic capacitor in order to increase its effective surface area. There is a d.c. etching method which applies a d.c. voltage with the aluminum foil as the anode and an a.c. etching method which applies an a.c. voltage between the aluminum foil and a carbon plate as the opposite electrode. Though deeply roughening the surface, the d.c. etching method has the problem that the mechanical strength is reduced. Though the surface is shallowly roughened in the a.c. etching process, it has problems in that aluminum is dissolved in the etching solution while only reducing the thickness. As a result it has low efficiency in that the effective area of the aluminum foil does not increase in proportion to the quantity of aluminum dissolved and on the contrary is likely to decrease from a certain point.